(1) Field of the Invention
The present invention relates to a solid-state imaging device that outputs a luminance signal in accordance with an amount of received light, and in particular to a solid-state imaging device, camera, and driving method for a solid-state imaging device that prevents the occurrence of black-crush in an image when strong light enters the device.
(2) Description of the Related Art
Recently, imaging devices which utilize solid-state imaging devices, such as consumer video cameras and digital cameras, have become common. Among such imaging devices, there are devices which employ amplifier-type image sensors as the solid-state imaging device.
While amplifier-type image sensors have excellent features such as low noise levels, they have a problem in that black-crush occurs in images when strong light enters the sensors. “Black-crush” refers to the phenomenon where, due to strong light, electric charge overflows from light-receiving elements and builds up in detection capacitors, which leads to a decrease in the amount of electric charge that can be stored; this in turn leads to parts in images which are supposed to be bright turning black.
Various technologies have been proposed conventionally as means to avoid this black-crush phenomenon (for example, see Patent Reference 1: Japanese Patent Application No. 2004-212263).
Patent Reference 1 discloses the following as a means to avoid black-crush: an MOS image sensor, which is an amplifier-type image sensor, and which includes an output unit that outputs, per unit cell, a luminance signal indicating the difference between a reset voltage and a read voltage when the read voltage falls within a predetermined range, and outputs, per unit cell, a luminance signal indicating increased intensity when the read voltage does not fall within the predetermined range. The image sensor described in Patent Reference 1 attempts to avoid the black-crush phenomenon through such a setup.
However, the circuit disclosed in Patent Reference 1 has the following problems.
First, in the circuit described in Patent Reference 1, a bypass transistor is provided for bypassing the clamp capacitor, and the read voltage is applied to the bypass transistor; when the bypass is applied to the gate terminal thereof and a drop in the read voltage caused by high-intensity light exceeds a predetermined voltage range, current flows in the subthreshold region of the bypass transistor, and a luminance signal indicating high intensity is outputted. However, the current in the subthreshold region is minute, and thus there are cases where variance occurs in the subthreshold current due to shifting and variance in the read voltage of the unit cell. Accordingly, the circuit described in Patent Reference 1 has a problem in that the operation for outputting the luminance signal that indicates the intensity is unstable.
Furthermore, with the circuit described in Patent Reference 1, the timing at which current flows changes in the case where the driving frequency, driving timing, and so on of the solid-state imaging device have been changed; therefore, there is a need to adjust the gate voltage of the bypass transistor and optimize the subthreshold current. In other words, the circuit described in Patent Reference 1 has another problem in that the configuration of a high-intensity judgment circuit must be changed each time the driving frequency, driving timing, and so on are changed.
Further still, with the circuit described in Patent Reference 1, in order to positively judge the intensity, the abovementioned predetermined range must be maintained against shifting in the read voltage caused by fluctuations in production processes, variation in the abovementioned subthreshold current, and so on; therefore, the luminance signal range is restricted. In other words, the circuit described in Patent Reference 1 has yet another problem in that the dynamic range of the luminance signal decreases so that the intensity can be judged while maintaining a constant margin for shifting, variance, and so on in the signal caused by fluctuations in production processes.
Accordingly, an object of the present invention, which has been conceived in light of the abovementioned problems, is to provide a solid-state imaging device and driving method thereof which, in response to the problem of black-crush occurring in an image when strong light enters the device, positively judges the occurrence of black-crush in a state in which a variance margin has been secured, without needing to change the configuration in response to changes in driving frequency or the like; and which can positively judge the occurrence of black-crush while maintaining the dynamic range of signals even amidst fluctuations in production processes.